1. Field of the Invention
The invention relates to a method and an apparatus for the cathode-side disposal of water and inert gas and/or the anode-side disposal of inert gas from a block having a number of fuel cells. A PEM fuel cell, where PEM is the abbreviation for polymer electrolyte membrane or proton exchange membrane, or an acidic or alkaline fuel cell, are possible fuel cell types in such a method and apparatus.
A fuel cell generally includes an electrically conductive current transformer plate, a cathode, an ion-conducting intermediate layer, an anode, and a further electrically conductive current transformer plate, which are stacked together in that order in the form of flat plates, and in which the anode, electrolyte and cathode form a membrane electrode unit (ME).
Fuel cells with such a construction are known, among other sources, from the Fuel Cell Handbook by Appelby and Foulkes, New York, 1989, and by the article by K. Strasser, entitled "Brennstoffzellen fur Electrotraktion" [Fuel Cells for Electric Traction], VDI Reports, No. 912, 1992, pp. 125-145, as well as German Published, Prosecuted Application DE-AS 27 29 640. Since the fuel cell is capable of converting chemically bound energy directly into electrical energy, it makes it possible for fuels such as hydrogen, natural gas and biogas to be converted into electrical energy at greater efficiency and with less environmental burden than the previously known conventional internal combustion engines, having an efficiency which is limited by the so-called Carnot process, were capable of doing.
A block of fuel cells is usually made up of alternatingly stacked together diaphragm-electrode units, gas chambers, cooling units and pressure cushions. Seals and possibly spacers are built in between those individual components. The various liquid and gas chambers of the block are supplied from axial channels through radial channels that extend through the seals. Such an axial channel extends at right angles to the plane of the stacked-together plate-like components of the fuel cell block. Such a radial channel extends correspondingly in the plane of the plates.
During operation of known fuel cell blocks, in particular with blocks formed of PEM fuel cells, the problem arises, for example, even when the anode side is supplied with industrially pure water and the cathode side is supplied with industrially pure oxygen, that water, which is created in the fuel cells as a result of the electrochemical reaction of hydrogen and oxygen, and inert gases such as nitrogen, carbon dioxide, and noble gases, are concentrated when supply gases are carried in circulation. Methods previously employed for disposing of the water and inert gas component or of the inert gas component from the cathode-side or anode-side gas mixture have been based on the cathode side on a superimposed oxygen circuit with a condenser, from which liquid water is continuously separated out. In the process, however, the inert gas is enriched continuously, causing the cell voltage and therefore the efficiency to drop. Even raising the flushing rate has no effect on the enrichment with inert gases, since it merely reduces the proportion of water. As the flushing rate increases, the increased capacity requirement for the condenser also markedly reduces the system efficiency. The flushing rate is the ratio between the discharged and the introduced volumetric flow of the anode-side or cathode-side gas mixture.
As the flushing rate increases, the capacity requirement of the condenser for humidified oxygen increases as well and thus decreases the efficiency. The aggressive media to be condensed, such as humidified, hot oxygen, which attack parts of the condenser, also increase the expense for maintenance as the condenser capacity increases. A problem which also exists is that in a fuel cell block that is integrated in a secure tank, the heat loss of the condenser must be dissipated. The relatively high noise level of the condenser must also be abated.